multi: use btcd's btcec/v2 and btcutil modules

This commit was previously split into the following parts to ease
review:
 - 2d746f68: replace imports
 - 4008f0fd: use ecdsa.Signature
 - 849e33d1: remove btcec.S256()
 - b8f6ebbd: use v2 library correctly
 - fa80bca9: bump go modules

channeldb/payments_test.go

@@ -8,7 +8,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcwallet/walletdb"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/kvdb"
@@ -19,7 +19,7 @@ import (
 )
 
 var (
-	priv, _ = btcec.NewPrivateKey(btcec.S256())
+	priv, _ = btcec.NewPrivateKey()
 	pub     = priv.PubKey()
 
 	testHop1 = &route.Hop{